1. Field of the Invention
The present invention relates to a precursor sol of aluminum oxide (sol as a precursor to an aluminum oxide film) and a method for manufacturing it and to a method for manufacturing an optical member, an optical member, and an optical system. More specifically, the present invention relates to a precursor sol of aluminum oxide that produces an optical coating relatively free of cracks and highly antireflective against light in a broad wavelength range including the visible range and to a method for manufacturing this sol as well as to a method for manufacturing an optical member, an optical member, and an optical system similarly advantageous.
2. Description of the Related Art
Antireflective structures having a fine structure smaller than the wavelengths in the visible range offer excellent antireflective properties against light in a broad wavelength range when the fine structure have an appropriate height and are arranged at appropriate pitches. A known way to produce such a fine structure is to apply a solution containing fine particles smaller than the visible wavelengths to a base and then form a coating containing a periodically distributed 100-nm to 250-nm fine structure.
There is also a technique in which fine processing equipment such as an electron-beam, laser interference, or semiconductor lithography system or an etching machine is used to form a pattern, and by this technique it is possible to produce a fine structure with controlled pitch and height and excellent antireflective properties (Japanese Patent Laid-Open No. 50-70040).
In another known method, a film of boehmite, i.e., an oxide hydroxide form of aluminum, is grown on a base until the film has an antireflective effect. This method produces an antireflection coating in the following way: An aluminum oxide film is formed on a base by vacuum coating or a liquid-phase (sol-gel) method followed by drying or baking, and the obtained film is treated with steam or immersed in hot water to form a fine structure of boehmite on the surface of the base (Japanese Patent Laid-Open No. 9-202649).
An antireflection coating produced using a fine structure of an aluminum compound has excellent antireflective properties owing to its very low vertical incidence reflectance and oblique-incidence reflectance. However, the antireflective properties of an antireflection coating based on a fine structure of an aluminum compound are sensitive to changes in the pitch and height of the fine structure.
In particular, its antireflective properties against obliquely incident light are acutely sensitive to the pitch and height of its fine structure. Given a fixed pitch of a fine structure, increasing the height of the fine structure leads to improved antireflective properties against obliquely incident light, but this requires the aluminum oxide film be thicker and have no or few cracks despite its increased thickness.
When a liquid-phase (sol-gel) method is used, the drying or baking operation following film formation produces heat and evaporates the organic solvent contained in the film, putting the film under tensile stress. The tensile stress becomes greater with increasing thickness of the film and as the measurement point is closer to the surface. It is therefore difficult to form a thick film on a base with a known precursor sol of aluminum oxide because the formed form easily cracks when dried or baked.